The feline sarcoma virus genome consists of three parts - a sarcoma specific portion of which two different types have been isolated, a common portion which represents sequences shared by all the subtypes of FeLV and FeSV, and a type specific common portion which represents gene(s) shared by FeLVA and FeLVC and FeSV. The sarc sequences are unrelated to RD114, mink leukemia, WSV, MSV, or other type C viral sequences. One type of sarc is predominant in the known natural FeSV isolates. The common portion shares some homology to mink virus but otherwise behaves similar to total FeLV cDNA sequences. The AC specific sequences do not have FeLVB sequences but are otherwise like FeLV common sequences. The sarcs are present in DNA in a single copy in all Felidae species tested; common sequences are present in multiple copies only in the Mediterranean cats; type specific sequences have mutated so far in horizontal transmission that no homology to normal cat DNA is seen. No FeSV sequences can be found in cat cellular RNA normally; however, virus negative lymphomas express common sequences variably but again no sarc or type specific sequences. Work is continuing to relate the FOCMA and natural lymphoma RNA to this common subset of FESV sequences.